One of our compilers for emscripten coerces all signaling NaNs to
quiet ones, so won't do any actual signaling. Anyone relying on them
to do so shall be disappointed, so it's better that they know about it
at compile-time - or, at least, have the ability to find it out.
Put the signaling NaN producers (and remaining (test) code using them)
under the control of a feature that's disabled when numeric_limits
claims double has no signaling NaN. Assume the bootstrap library
doesn't need signaling NaNs. Sadly, until C++20 <bit>, there's no
contexpr way to test that alleged signalling and quiet NaNs are
actually distinct.
Added some auto-tests for signaling NaN, including that it's distinct
from quiet NaN. Any platform on which the last fails should disable
this feature.
Task-number: QTBUG-77967
Change-Id: I57e9d14bfe276732cd313887adc9acc354d88f08
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
4bd6cd1992